BACKGROUND: The number of individuals who are diagnosed with attention-deficit hyperactivity disorder (ADHD) during adulthood has increased in recent years. However, there is still no decision aid (DA) to help adults newly diagnosed with ADHD make decisions regarding further treatments. OBJECTIVE: This study aimed to describe the development process of a DA for adults newly diagnosed with ADHD and its field testing during the shared decision-making (SDM) process in a clinical setting. METHODS: The development process involved the creation of a DA prototype using the International Patient Decision Aid Standards criteria and revision of the prototype through the stakeholders' reviews. The field testing of the DA compared scores before and after the SDM process on the service users' knowledge scale, decisional conflict scale and the Conners Adult ADHD Rating Scales. RESULTS: The developed DA contained options of watchful waiting with own coping skills and pharmacological treatment, which consisted of several kinds of drug options. Fifteen adults newly diagnosed with ADHD participated in the field testing. The participant decision-making quality outcomes such as their knowledge and decisional conflict improved after the SDM process. ADHD severity did not change. CONCLUSION: A DA for adults with ADHD was systematically developed following the international criteria. Field testing indicated that the DA could serve as a tool to facilitate the SDM process. Further research on this DA is necessary before its routine implementation. PATIENT OR PUBLIC CONTRIBUTION: During the development process of the DA, the service users who had already been diagnosed with ADHD reviewed the DA prototype and provided feedback, which improved the final version of the DA.